The moisture content of sheets housed in the sheet supply cassette and similar locations varies depending upon the installation environment of an image forming device and the season. If the moisture content of the sheets is greater than a specific value, many types of problems have occurred, such as voiding and other transfer defects, and defects in separation from the photosensitive drum.
As a countermeasure to this, conventionally heaters have been provided within the image forming device or sheet supply cassette, etc., in order to reduce the moisture content of the sheets by the heat of the heaters. However, although this countermeasure is effective for sheets in the sheet supply cassette within the image forming device, it is not effective for sheets on the manual feed tray outside the main body of the image forming device.
Therefore image forming devices as described below are known.
The image forming device A as shown in FIG. 3 includes a scanner unit B as image reading means for reading the image information of original documents, an image forming unit C as image forming means, and a sheets deck D. The scanner unit B includes a scanning system light source 201, a platen glass 202, a document pressure plate 203 that can open and close with respect to the main body of the image forming device, a mirror 204, a lens 205, a light receiving element (photoelectric transducer) 206, and an image processing unit (not shown in the drawings).
A book or sheet is placed face down on the platen glass 202, then with the document pressure plate 203 pressing against the rear surface a reading start key (not shown in the drawing) is pressed. This causes the scanning system light source 201 to scan the underside of the platen glass 202 in the direction of the arrow “a” to read the image information on the document surface. Document image information read by the scanning system light source 201 in this way is processed in the image processing unit, converted to electrical signals, and transmitted to a laser scanner 111.
A sheet supply cassette (internal loading unit) 1 is provided below the image forming unit C. The sheet supply cassette 1 comprises a lower cassette 1a and an upper cassette 1b forming a single sheet supply unit. Here three sheet supply units, U1, U2, and U3, are provided. The highest sheet supply unit U1 is detachably installed on the main body of the device A, and the other two sheet supply units U2, U3, are detachably installed on the sheet deck D.
Sheets contained in cassettes 1a, 1b are fed out by a pick up roller 3, and after separating and feeding one sheet at a time by the cooperative action of a feed roller 4 and a retard roller 5, one sheet is fed by a pair of feed rollers 104, 105 as far as a pair of stationary register rollers 106. Then the sheet is fed to a transfer unit between a photosensitive drum 112 and a transfer roller 115 at a predetermined timing by the rotation of the pair of register rollers 106.
Also, the side of main body of the device A includes a manual feed tray (external loading unit) 10. Sheets S loaded in the manual feed tray 10 are fed by a manual sheet supply roller 11, and transported to the pair of stationary register rollers 106.
The image forming unit C includes a photosensitive drum (image carrying body) 112, an image writing optical system 113, a developer 114, and a transfer roller 115. The surface of the photosensitive drum 112 is uniformly charged by a primary charger (not shown in the drawings). The surface of the photosensitive drum 112 is then scanned by the image writing optical system 113 with laser light corresponding to the image information emitted from the laser scanner 111, to form a latent image. The latent image is developed into a toner image by the developer 114, and transferred to the sheet S passing through the transfer unit by the transfer roller 115.
The sheet S onto which the toner image is transferred is transported to a fixing device 118 by a transfer belt 117. Then the toner image is heated and subject to pressure while passing through the fixing device 118 and fixed onto the surface of the sheet. After fixing, the sheet S is output to a sheet output tray 120 outside the image forming device by a pair of output rollers 119.
To form an image on both sides of the sheet S, the sheet S is held by the pair of output rollers 119 after it has passed the fixing device 118, and when the rear edge of the sheet S has passed a branch point 207 the pair of output rollers 119 reverses. As a result of this the sheet S is temporarily placed in a double-sided tray 121. Then the sheet S is transported by the pair of feed rollers 104, 105 to the stationary pair of register rollers 106. Then the image is formed on the second side of the inverted sheet S in the same way as described above, and output to the sheet output tray 120.
The image forming device has the following characteristics.
A humidity sensor 300 is provided within the main body of the image forming device A. The humidity sensor 300 is provided within the image forming device A on the side wall that contains the manual feed sheet supply unit, and measures the humidity of air taken in through slits 301 (FIG. 4) on the side wall. When the manual feed tray 10 is open (FIG. 3) the humidity sensor 300 is directly above the manual feed tray 10, so the humidity of the air that affects the sheets S in the manual feed tray can be measured.
The humidity information measured by the humidity sensor 300 is sent to a CPU (control means) 400. Then based upon the humidity information the CPU 400 sets the transfer bias to the sheet S and the separation voltage from the photosensitive drum 112 to the sheet S.
As described above, according to the image forming device the moisture absorption of the sheets is measured, then the transfer bias and the separation voltage are set in accordance with the moisture absorption. Therefore image forming can be carried out without transfer and separation defects.
In this image forming device humidity is measured by a humidity sensor, which has the advantage that measurements are highly accurate, but it has the disadvantage that the cost is high. In other words, for setting the transfer bias and separation voltage it is sufficient to know the humidity approximately, so a cheaper mechanism for detecting the humidity is desirable.
The present invention is proposed based upon the conventional situation as described above, and it is an object to provide an image forming device at low cost that is capable of forming good images without transfer and separation defects.